John Barrett wrote:
I probably know the answer to my question before I ask but I am running my TSIO 550 lean of peak and I want to be sure I am using adequate precautions and am doing the best for the engine.
To that end, if my engine baffling is not as good as it could be and I intend to continue to improve it, will a GAMI test now resulting in, say, a 1 gph difference in FF for peak EGT among the 6 cylinders be useful for me to send to Tornado
Alley for GAMIjectors? In other words, if I improve my baffling after I get George Braly’s fix, will that investment still be relevant?
John,
I’ll take a shot at this and defer to/welcome other comments from the experts.
First of all, baffling changes will not significantly affect EGTs…and vice versa. EGTs are a closely-related
measure of the power being produced by the cylinder… and a host of other factors related to combustion. CHT’s are a measure of how much heat is being transferred to the cylinder…and how much is being removed by cooling air. Again, lots of other factors affect
cylinder temperatures, and while logic would tell you that high EGTs will yield high CHTs, the relationship contributes little as compared to other factors. So with this in mind, consider these points.
To run a TSIO 550 lean of peak, it must be operated below 75% power. My personal rule is to run it at or below
65% of rated power. Engine component heat, and particularly cylinder heat is the limiting factor, and the TSIO-550 can produce almost 100% of rated power up into the teens if you leave the controls full forward. You’ll find power setting info in the engine
manual, and if you have a modern engine monitoring system you can probably program this into the monitor to give a direct readout of horsepower or a percentage of max rated… With or without this, you need to derive this information from total fuel flow (LOP
only) or a combination of MP and RPM, but it’s important that you know--and operate the engine at lower power settings--if you want to run LOP. (NOTE: Some experts such as John
Deakin “The Pelican” say that you can run these engines LOP at 100% of rated power AS LONG AS YOU CAN KEEP THEM COOL, but then acknowledge that as a practical matter this can’t be done with current production aircraft engines…they
produce too much heat for the available cooling systems at high power settings.
When you run rich of peak, some of the fuel is used directly for cooling and passes through the engine without
burning. As a matter of course, it cools by evaporation, and leaves nasty lead and other deposits behind…on plugs, valves, etc. So when you run LOP and eliminate this source of cooling, you must offset it by either reducing power (total heat produced) or increasing
cooling (better baffling). So as you lean the mixture toward max cylinder EGT, you’re reducing the amount of fuel available for cooling and the CHT’s go up. With most Continental engines, this doesn’t happen at the same fuel flow for all cylinders, that is,
one cylinder reaches peak EGT at a given fuel flow (around 16-17 GPH if you’re running about 65% power) as you lean, while the other cylinders are still rich of peak EGT. You need to note the fuel flow when the first cylinder reaches peak EGT. As you continue
to lean, the EGT (and CHT to a lesser extent) of the first to peak starts going down, while the remaining cylinder EGTs (and CHTs) continue to go up. Eventually, all cylinders reach peak EGT and start decreasing. Note the fuel flow when the last cylinder reaches
peak EGT and just starts to decrease. The difference in fuel flow between first and last cylinder EGT to peak is the “GAMI lean test.” Ideally, the fuel flow between first and last should be less than 1 GPH; use of GAMIs can tune this to less than half a gallon
per hour, BUT NOTE THAT THIS IS OPTIMIZED FOR ONLY ONE POWER/RPM setting, so you should test/set up the engine at the normally-expected cruise power setting.
Here’s the rub: You need to be sure all cylinders are at least 30 degrees below their peak EGT when operating
the engine in cruise power, and this is critically important when LOP. Ideally, all cylinders are all operating at about the same amount below their individual peak EGTs, but the actual temperatures are not significant…it’s the peak value and delta below--for
each cylinder--that’s significant. If the fuel flow is not well balanced among the cylinders, the first to peak EGT will get so lean by the time that you get the last to 30 degrees below peak EGT that the first to peak starts
mis-firing. My cylinders start mis-firing at about 120 degrees LOP as a reference point. Mine are well enough balanced (no GAMIs) that I can operate the richest cylinder at 60 degrees LOP and the leanest
cylinder (first to peak when leaning) will remain below 100 degrees LOP. That equates to about a 0.5 GPH fuel flow difference FOR MY ENGINE…but this varies a lot from engine to engine, and with the power/RPM setting.
BTW, the TIT’s ARE CRITICAL, and you’ll want to watch these while you’re fiddling with the EGTs. Continental
says the max TIT is 1750 and the max continuous is 1650. I am more conservative, and have a “yellow warning” set at 1550 and a “red alarm” set at 1650. As you lean the engine, TIT’s will increase with EGTs, and will continue to increase for a short time after
the first EGT peaks are reached, and then start decreasing. I normally see about 1500 on my TITs in cruise flight; I lean the mixture more if they go above 1550 in cruise flight.
Continental says the max CHT is 460 degrees. I am pretty convinced you’ll cook your cylinders if you operate
them anywhere close to that in normal cruise flight. I have a “yellow warning” set at 400 and a “red alarm” at 420. I only see these temps during high power climbs between about 12,000 and 16,000, when the engine is still producing lots of power but the air
is starting to thin/provide fewer molecules to carry away the heat. In any case, when my CHT’s get to 420, I increase the fuel flow (low boost if not already on) , richen the mixture control (climbing ROP anyway) and/or increase airspeed. As a last resort,
I’d pull the power back, but have never had to do this…yet.
As to CHTs during cruise flight. Get the fuel flow to the cylinders balanced first…effectively you are making
each cylinder produce approximately the same amount of power (yields a smoother running engine)--and heat. Remember that this will be optimized only for a small range of power/RPM settings. Then go to work on balancing the CHTs. This is (at least theoretically)
much easier than balancing the fuel flows and EGT peaks because you can do many things to change the cooling air flow across the cylinders--and that’s the only variable IF you’ve balanced the fuel flow/cylinder power output. Working CHT’s first just won’t
yield good results…you’ll wind up doing it over and over…
Back to precautions you asked about:
1)
If you’re running LOP, be sure you have the engine producing less than 75% of its rated power; below
65% if you’re conservative.
2)
Be sure that every cylinder is LOP and that the hottest EGT is at least 30 degrees below the cylinder’s
max EGT (without consideration of the actual value, nor the values of any other cylinder)
3)
Keep the TITs below 1650 degrees; ideally below 1550.
4)
Keep the CHT’s below 420; ideally below 400 (If you do the above things, they will likely be in the
300-340 degree range)
As to GAMI’s: if you can get the engine to run smoothly, without
mis-firing, with the richest cylinder (last to max EGT as you lean) at least 30 degrees LOP you probably don’t need GAMIs. Older Continentals won’t do this; the newer ones have a Continental version of GAMIs (flow-matched injectors)
and are better, but not as good/close as the fine-tuned GAMIs. A second measure of evaluation is fuel flow. If the fuel flow between the first and last cylinders to peak is greater than 0.5 GPH, GAMIs will improve this, and likely result in a smoother-running
engine.
LOTS of stuff here; hope it helps both setup and understanding…
Bob